The present invention relates to apparatus for providing a magnetic field and, more particularly, to a magnet having a superconductive coil for providing a region having a very homogeneous magnetic field of relatively high flux density.
One of the main areas of application for a magnet providing a region having a substantially homogeneous magnetic field with high flux density is in equipment making use of the phenomenon of nuclear magnetic resonance (NMR) imaging. One such piece of equipment for medical diagnosis provides information comparable to that provided by a computerized axial tomography scanner but without subjecting the patient undergoing examination to potentially harmful X-rays. Additionally, an NMR imaging system offers other advantages over a computerized axial tomography scanner in that the operation of the former is not affected by the presence of bone. Ultimately the patient can safely remain in place long enough, and the NMR imaging system can be designed to have sufficient resolution to allow observation of the body's take up of a drug.
It has been recognized that the magnetic field provided by prior art simple solenoid coils is not usually sufficiently homogeneous for NMR imaging. One prior art magnet for NMR imaging incorporated a superconductive solenoid coil for generating a field of the desired strength, and further included a number of separate room temperature correction coil windings precisely positioned at predetermined locations inside the warm bore along the length of the solenoid to correct for nonuniformity in field strength. Available mathematics permit computation as to the the number of turns and the identification of the precise locations of these correction coils to achieve the desired field homogeneity at the center of the magnet bore, which must have a deviation in field strength of approximately twenty-five parts per million for proper NMR imaging.
In another prior art magnet, a superconducting solenoid is employed having a field-shaping member made of a metal having high magnetic saturation. The field-shaping member functions to prevent production of localized regions of high magnetic filed strength at inner points of the superconducting winding. The field shaper interacts with the magnetic filed lines in the localized region and directs the field lines to form a resultant substantially uniform strong field along the axis of the solenoid. Reference may be made to U.S. Pat. No. 3,177,408 for a further description of the structure and operation of this magnet.